In general, the present invention relates to techniques for determining electrical size, as well as the physical design/structure and other characteristics, of electromagnetic (EM) radiation sources (or simply referred to as, antennas) that operate in a frequency range up to about 5 GHz. The novel technique and associated xe2x80x9celectrically smallxe2x80x9d radiator structures described herein allow radiation/waves to be xe2x80x98launchedxe2x80x99 as a generally directed beam and radiate away from the radiator source rather than remaining in proximity to the structure (as xe2x80x9cstanding energyxe2x80x9d) when operating. More particularly, the instant invention relates to electrically small, wideband radiator structures for radiating EM waves as well as a novel method of producing EM waves and associated novel techniques for producing novel electrically-small radiator/antenna designs, such that the source-associated standing energy, i.e. the energy that returns from the radiated field to the structure to affect operation, is minimal. According to the novel design technique of the invention, optimally the source-associated standing energy for a fully-optimized xe2x80x98perfectxe2x80x99 radiator structure of the invention (i.e., one that behaves identically as predicted by mathematical theory), would be zero. To produce designs having minimal source-associated standing energy, the technique of the invention incorporates the identification of a solution to generally satisfy a unique expression derived by the applicants hereof. This unique expression utilizes the time-dependent Poynting theorem (rather than the conventionally-used complex Poynting theorem, the frequency-domain solutions for which are missing important antenna phase information) and takes into account three numbers/expressions in specifying time-varying power of a radiating antenna structure rather than just two numbers/expressions, as has conventionally been done to create solutions using the complex Poynting theorem.
The application of the novel techniques of the invention leads to the design of novel radiator structures, each structure preferably having at least four dipole moments arranged as dipole pairs with an overall electrical size, k*a, with a value less than xcfx80/2. Each dipole pair is configured to have at least a magnetic dipole element, and preferably also an electric dipole moment, the dipole pairs oriented in such a way that: the divergence of the Poynting vector of the system of two pairs of dipole moments with respect to xe2x80x98retarded timexe2x80x99 is a small, or negligible value (and, in an optimal case, this divergence value is zero). Although considered electrically small, surprisingly these novel structures readily emit waves with longer wavelengths (such as are encountered in wireless communications, radar detection, microwave technology devices, and medical device technology) at lower frequencies (throughout the electromagnetic wave Radio Spectrum and below, generally targeting frequencies less than 5 GHz) as non-reciprocal, wideband devices.
The low frequency radiator structure designs of the invention, unlike any currently in use, can be sized with a relative electrical length smaller than ka≈xcfx80/2, where the physical dimension xe2x80x9caxe2x80x9d used throughout is that identified by Chu (1948), and indeed sized as small as ka≈xcfx80/2000 (i.e., up to 1000 times smaller than any currently in operation); and such a structure may readily be configured up to 10,000 times smaller than any conventional antenna, or where ka≈xcfx80/20,000. For further background reference, see Chu, L. J. Physical limitations of omni-directional antennas, J. Appl. Phys., 19, 1163-1175, 1948, for an analysis of one-dimensional multipolar sources of only electric dipoles (TM) fields. In his research, Chu (1948) provided a physical interpretation of dimension a by constructing the smallest possible circumscribing sphere having a radius xe2x80x9caxe2x80x9d that fully contained the radiating source to then calculate the integral of the complex Poynting vector over that surface. Traditional and current antenna design practices lead designers to build extremely long structures to emit electromagnetic waves at selected frequencies, for example, the dimension a of an electric dipole antenna that operates at a frequency of 1 MHz would be on the order of 150 meters, and a 1.0 GHz dipole antenna for wireless communications would be approximately 15 cm in length. Whereas, using the novel technique of the invention allows one to produce EM waves using novel radiator structures sized on the order of 0.150 m (at 1 MHz) and 0.015 cm (at 1 GHz) long, respectively.
The historical difficulty in directing scientific research toward the exploration of building low Q, electrically small antennae stems from the conventional use of frequency domain mathematics to describe operational performance. According to accepted definitions, reactive power in electrical circuits is in time quadrature with the real power and its magnitude is 2xcfx89 times the energy that oscillates twice each field cycle between the source and the circuit, where xcfx89 is the radian frequency of the field. It is widely believed that this statement applies to power in radiation fields, differing only in that energy oscillation is between the source and the fields. It is commonly accepted that, for a closed volume in space, the real part of the surface integral of the complex Poynting theorem is equal to the time-average output power and the imaginary part is proportional to the difference between the time-average values of electric and magnetic energy within the volume. By way of review: The Poynting vector was defined long ago in the late-1800""s in connection with the flow of electromagnetic power through a closed surface as xe2x89xa1Exc3x97H VA/m2, or W/m2; J. H. Poynting, xe2x80x9cOn the transfer of energy in the electromagnetic field,xe2x80x9d Phil. Trans. Royal Society, 175, 343, 1884. For further general background information and explanatory figures on the theorem of Poynting, particularly the simplification of the complex Poynting for the time-average Poynting theorem, see the reference C. T. A. Johnk, Engineering Electromagnetic Fields and Waves, John Wiley and Sons, Inc., New York, pp. 385-402 (chapter 7), 1988.
In their pursuit to more-closely study power in radiation fields in earlier work (see Grimes, D. M., and C. A. Grimes, xe2x80x9cPower in modal radiation fields: Limitations of the complex Poynting theorem and the potential for electrically small antennas,xe2x80x9d Journal of Electromagnetic Waves Applications, vol. 11, pp. 1721-1747, 1997), two of the applicants hereof rigorously analyzed power in sinusoidal steady state radiation fields and identified that for certain antenna designs the conventional practice to define reactive power as the imaginary part of the surface integral of the complex Poynting vector (which allows for a more straight-forward calculation thereof) causes a loss of very important information about the radiation source""s properties. The authors, Grimes and Grimes (1997) instead found that in order to find solutions that correspond better with what is actually happening in the fields around an antenna, use of the time-dependent Poynting theorem (TDPT) characterizes power in a sinusoidal field with three important values. In an effort to simplify notation within their mathematical expressions, Grimes and Grimes (1997) introduced the variable tr=txe2x88x92"sgr"/xcfx89 (which they refer to simply as xe2x80x9cretarded timexe2x80x9d where: xcfx89=radian frequency, "sgr"=kr, k=wave vector, and r=radial distance from source).
In their 1997 publication, applicants Grimes and Grimes point out a fatal flaw in the premises (particularly, the concept applied regarding power in a radiation field) on which commonly accepted proofs concerning the behavior of the radiative Q of a radiation source (antenna) have been conventionally based. More particularly, these commonly accepted proofs lead to the conclusion that, in the limit as the product k*a goes to zero, the radiative Q of a radiation source (e.g., an antenna) goes to infinity. It is well known, that the standing energy adjacent an imperfect conductor causes power loss through surface current on the conductor. From these commonly accepted proofs concerning the behavior of the radiative Q of a radiation source, convention has it that, as the product k*a decreases for a dipole antenna, the antenna acts less as a generator of EM radiation and more like an energy-storage device (such as a capacitor). Thus, the following relationship has been universally applied to the design analysis of dipole antennae: The radiation-field standing energy in proximity to the antenna structure varies as the inverse cube of k*a. And this has lead to the following prevailing accepted conventional design criteria for antennae: The product of the wave number k of the radiation (where k=2xcfx80/xcex) and xc2xd of the largest physical dimension of the radiation source (or, a, the value Chu (1948) defined) can be no less than approximately xcfx80/2, and thus an operational antenna can be no smaller than a=xcex/4 (i.e., no less than one-fourth of the wavelength being radiated by the antenna).
Radiative Q is commonly used in describing the energies associated with antennas. A more-detailed explanation of Radiative Q is set forth below. The identification of the flawed premises upon which conventional antenna design practices are based influenced the applicants hereof to further analyze known ways to calculate Q for a radiation source and develop a novel method of determining Q based upon the time-dependent Poynting theorem that incorporates three necessary power expressions to describe the source-associated standing energy (including the two expressions found within the complex Poynting theorem plus the modal phase angle). This, in-turn, led to the ingenious techniques and novel electrically small radiating structure designs and methods of the instant invention, which effectively radiate as multi-element EM sources with a k*a product less than xcfx80/2, unlike conventional EM sources currently in use.
The new electrically small radiator structures and method of producing an EM signal and generally-directed beam as described herein, are suitable in operation with a wide range of EM wave generation, phase shifting, power splitter, circulator, and oscilloscope equipment to produce such signals. In the spirit of the many radiator designs contemplated hereby, the innovative, simple, and effective radiator structures and methods are suitable for use in a variety of environments allowing the structures to be tailored and installed with relative ease into available equipment. None of the currently-available EM radiating systems take advantage of the novel techniques identified herein to produce multi-element radiator structures that can be incorporated along with micro-components into associated microcircuits, as will be further appreciated.
It is a primary object of this invention to provide a multi-element electrically small radiator structure for radiating electromagnetic waves. This structure having an electrical size, or k*a product, of preferably less than xcfx80/2 and greater than, say, xcfx80/20,000, and configured to have at least a first and second magnetic dipole element. Such a structure may further have two or more pairs of dipole moments, each pair comprising a magnetic and electric dipole moment. The pairs of dipole moments are preferably oriented such that the following is generally satisfied: a divergence of the Poynting vector of the pairs with respect to retarded time, namely ∇|tRxc2x7N, has a value less than 1.0. Further, the magnetic dipole moments of each pair are preferably oriented generally in parallel with a respective electrical dipole moment, with the dipole pairs oriented generally orthogonally with respect to each other. The voltage across each dipole pair is preferably in phase-quadrature, and the pairs can be separately fed using a single power source. It is a further object to provide a method of producing an electromagnetic signal, which can be a generally-directed EM beam, with an electrically small radiator structure such as any structure produced according to the novel technique of the invention.
Certain advantages of providing the new radiator structures and associated new methods, as described and supported hereby, include the following:
(a) The novel radiator structures and method allow for a generally directed beam of energy to be emitted from an electrically small structure, while minimizing the source-associated standing energy remaining in proximity to the structure, at lower frequencies (for example, 5 GHz and below).
(b) Versatilityxe2x80x94The invention can be used for sending lower-frequency EM signals (in turn, having longer wavelengths) over great distances, if necessary, using relatively small, non-reciprocal transmit-devices operational in a wide range of environments and applications. For example: in wireless/cellular communications, for sending information gathered about an area (e.g., to study the ocean floor, in aircraft and submarine radar obstacle detection, and in ground penetrating radar applications), in medical applications (e.g. directed-beam heating/removal of tumors, malignant tissue, cysts, etc.), in automatic manufacturing processes (e.g., auto-sensory equipment to detect whether a component is properly oriented and detecting surface roughness), and so on.
(c) Simplicity of usexe2x80x94The simplified design technique of the invention can be used to design many different types of suitable specific xe2x80x98electrically smallxe2x80x99 structures that efficiently operate at lower frequencies; the technique can be applied to a wide variety of elements able to effectively operate as electric-magnetic dipole pairs to generally satisfy design criteria specified herein. Furthermore, the new radiator structures and associated methods can be installed/hardwired/incorporated into, and readily operational with, existing radar, telecommunications, and product manufacturing equipment, plus inter-connected to existing computer systems (whether with UNIX-, LINUX-, WINDOWS(copyright)- WINDOWS NT(copyright), DOS, or MACINTOSH(copyright)-based operating systems) with relative ease.
(d) Design Flexibilityxe2x80x94Producing a radiator structure according to the invention using the novel design techniques/guidelines described herein, allows for fabrication of many different structures of a variety of shapes using many different suitable materials (depending upon the environment in which the antenna structure of the invention is intended to operate); including i) a compound antenna structure composed of two pairs of loop-wire structures (these two structures preferably electrically-insulated by suitable means, such as providing a spacing or coating the structure at a potential point of contact with a dielectric material), ii) microelectronic conductive elements oriented and fabricated according to well known microcircuit fabrication techniques such that the divergence of the Poynting vector of the system of two pairs of dipole moments with respect to xe2x80x98retarded timexe2x80x99 is small or negligible, iii) a membrane filled with a conductive gel-substance/plasma and a voltage source therewithin such that the divergence of the Poynting vector of the system with respect to xe2x80x98retarded timexe2x80x99 is small or negligible.
(e) Applicationsxe2x80x94The novel use of the time-dependent Poynting theorem to analyze the operation of electrically small antenna structures at lower-frequencies, after identifying flaws in current design practices, in concert. with using newly-identified conditions, give antenna design engineers not only a valuable novel technique of producing electrically small antennas but also a tool box full of new design structures for operation at lower-frequencies.
(f) Beam Directivity and Performance of an Array of Structuresxe2x80x94The novel technique for producing electrically small low Q antennas, the radiator structures produced thereby, as well as the method of producing an EM signal, are applicable to arrays of low Q radiator structures constructed according to the invention and arranged according to known antenna array factors to produce a system with a highly directed beam.
Briefly described, once again, the invention includes an electrically small radiator structure for radiating electromagnetic waves. The structure has an electrical size, k*a, with a value between xcfx80/20,000 and xcfx80/2 and is configured to have at least a first and second magnetic dipole element. Further distinguishing features of the invention: The dipole elements are preferably oriented such that a source-associated standing energy value for the structure, or WdS(tR), is low, and each of the elements is in communication through a feed circuit to at least one power source. A structure of the invention can be constructed such that a Radiative Q value therefor will generally be less than ⅓(k*a)3. The structure can have first and second dipole pairs, each comprising an electric dipole element and a magnetic dipole element; both pairs can be connected through a feed circuit to at least one power source. The dipole pairs are preferably generally electrically-insulated from each other. Further distinguishable, the first pair is preferably oriented orthogonally with respect to the second pair, a voltage across the first pair and a voltage across the second pair are in phase-quadrature with a radiated power from each pair being generally balanced, and the multi-element structure is operational at a frequency below 5 GHz. According to novel design techniques of the invention, the pairs of dipole moments can be oriented such that the following is generally satisfied: a divergence of the Poynting vector of the pairs with respect to retarded time, namely ∇|tRxc2x7N, has a value less than 1.0.
Also characterized herein is a method of producing electromagnetic waves using an electrically small radiator structure. The method comprises configuring the structure to have at least a first and second pair of dipole moments and an electrical size, k*a, with a value between xcfx80/20,000 and xcfx80/2; and powering a first feed area of the first pair and a second feed area of the second pair with at least one source operating at a frequency to radiate the waves. Features that further distinguish the invention from conventional methods: Forming a first elongated member into the first pair which includes a magnetic and electric dipole element and forming a second elongated member into the second pair which also includes a magnetic and electric dipole element, and electrically insulating the dipole pairs; orienting the pairs such that the following is generally satisfied: a divergence of the Poynting vector of the pairs with respect to retarded time, namely ∇|tRxc2x7N, has a value less than 1.0; orienting the pairs such that (a) a dipole moment axis of the first electric dipole element is generally in parallel with a dipole moment axis of the first magnetic dipole element, (b) a dipole moment axis of the second electric dipole element is generally in parallel with a dipole moment axis of the second magnetic dipole element, and (c) the first pair is orthogonal with respect to the second pair; and generating electromagnetic energy with a single source and passing it through a feed circuit electrically connected to a first feed area of the first pair and a second feed area of the second pair.